Enamel composition, method for preparing enamel composition, and cooking appliance

ABSTRACT

An enamel composition, a method for preparing an enamel composition, and a cooking appliance are provided. The enamel composition may include 15 to 50 wt % of phosphorus pentoxide (P 2 O 5 ); 5 to 20 wt % of one or more of lithium oxide (Li 2 O), sodium oxide (Na 2 O), or potassium oxide (K 2 O); 1 to 5 wt % of one or more of sodium fluoride (NaF), calcium fluoride (CaF 2 ), or aluminum fluoride (AlF 3 ); 1 to 35 wt % of one or more of magnesium oxide (MgO), barium oxide (BaO), or calcium oxide (CaO); and 5 to 30 wt % of one or more of manganese dioxide (MnO 2 ), molybdenum trioxide (MoO 3 ), bismuth oxide (Bi 2 O 3 ), or nickel oxide (NiO). The enamel composition may be cleaned without being putting it into water.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of Korean Patent Application No. 10-2019-0021143, filed in Korea on Feb. 22, 2019, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND 1. Field

An enamel composition, a method for preparing an enamel composition, and a cooking appliance are disclosed herein.

2. Background

Enamel is a substance where a glass glaze is applied onto a surface of a metallic plate. Ordinary enamel is used for cooking appliances, such as microwave ovens and ovens, for example. Cooking appliances, such as electric ovens and gas ovens, for example, are devices that cook food or other items (hereinafter, collectively “food”) using a heat source. Contaminants, for example, produced during cooking, are attached to an inner wall of a cavity of a cooking appliance. Accordingly, the inner wall of the cavity needs to be cleaned. Enamel is coated on a surface of the inner wall of the cavity of the cooking appliance, for example, and helps remove contaminants attached to the cooking appliance easily. Among the technologies for readily cleaning the inner wall of a cavity, a process of pyrolysis (thermal decomposition) by which contaminants are burned to ashes at high temperatures is widely known. Enamel compositions including components, such as phosphorus pentoxide (P₂O₅), silicon dioxide (SiO₂), and boron oxide (B₂O₃), for example, are known as an enamel composition to which the process of pyrolysis can be applied.

Even though an enamel composition includes P₂O₅, SiO₂, B₂O₃, and group I-based oxides, contaminants of poultry fat or monster mash may be removed after being soaked in water for a certain period of time. Additionally, as P₂O₅, and group I-based oxides are included in enamel compositions, durability of a calcinated enamel composition may decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described in detail with reference to the following drawings in which like reference numerals refer to like elements, and wherein:

FIG. 1 is a front perspective view of a cooking appliance according to an embodiment;

FIG. 2 is a partial enlarged cross-sectional view of a portion of an inner surface of a cavity of the cooking appliance in FIG. 1 ;

FIG. 3 is enlarged cross-sectional view of a portion of an inner surface of a door of the cooking appliance in FIG. 1 ; and

FIG. 4 is a flow chart of a method for preparing an enamel composition according to an embodiment.

DETAILED DESCRIPTION

Enamel Composition

An enamel composition according to embodiments may include 15 to 50 wt % of phosphorus pentoxide (P₂O₅); 5 to 20 wt % of one or more of lithium oxide (Li₂O), sodium oxide (Na₂O), or potassium oxide (K₂O); 1 to 5 wt % of one or more of sodium fluoride (NaF), calcium fluoride (CaF₂), or aluminum fluoride (AlF₃); 1 to 35 wt % of one or more of magnesium oxide (MgO), barium oxide (BaO), or calcium oxide (CaO); and 5 to 30 wt % of one or more of manganese dioxide (MnO₂), molybdenum trioxide (MoO₃), bismuth trioxide (Bi₂O₃), or nickel oxide (NiO).

P₂O₅ is a component that forms an alkali phosphate glass structure. P₂O₅ is also a glass former that helps addition of a large amount of transition metal oxides into an enamel composition, and helps water to permeate between an enamel surface and a contaminant, such that the contaminant is easily removed. P₂O₅ is contained in a range of 15 to 50 wt %. When more than 50 wt % of P₂O₅ is included, the enamel composition is hardly glazed, and thermal properties of the enamel composition may be deteriorated. Additionally, when less than 15 wt % of P₂O₅ is included, an amount of added transition metal oxides is reduced. Thus, a cleaning performance may be deteriorated.

Li₂O, Na₂O, and K₂O improve a cleaning performance of an enamel composition. One or more of L_(i2)O, Na₂O, or K₂O are contained in the enamel composition in a range of 5 to 20 wt %. When more than 20 wt % of the one or more of Li₂O, Na₂O, or K₂O is included, the coefficient of thermal expansion of glass may be extremely increased. Accordingly, a coating performance may be deteriorated. When less than 5 wt % of the one or more of Li₂O, Na₂O, or K₂O is included, a cleaning performance may be deteriorated.

NaF, CaF₂, and AlF₃ are components that control surface tension of an enamel coating layer and improve surface properties of the enamel coating layer. One or more of NaF, CaF₂, or AlF₃ are included in the enamel composition in a range of 1 to 5 wt %. When more than 5 wt % of the one or more of NaF, CaF₂, or AlF₃ is included, thermal properties may be deteriorated. When less than 1 wt % of the one or more of NaF, CaF₂, or AlF₃ is included, surface properties of the enamel coating layer may be deteriorated.

MgO, BaO, and CaO are components that improve adhesion between the enamel coating layer and a base metal plate. One or more of the MgO, BaO, or CaO are contained in the enamel composition in a range of 1 to 35 wt %. When more than 35 wt % of the one or more of MgO, BaO, or CaO is included, a cleaning performance may be deteriorated. When less than 1 wt % of the one or more of MgO, BaO, or CaO is included, adhesion between the enamel coating layer and the metal plate may be reduced. Thus, glass stability may be reduced.

MnO₂, MoO₃, Bi2O₃, and NiO function as a catalyst on a surface of the enamel coating layer. Accordingly, MnO₂, MoO₃, Bi2O₃, and NiO easily disconnect the surface of the enamel coating layer and the contaminant. One or more of MnO₂, MoO₃, Bi₂O₃, or NiO are included in a range of 5 to 30 wt %. When more than 30 wt % of the one or more of the MnO₂, MoO₃, Bi₂O₃, or NiO is included, the enamel composition is hardly glazed and thermal properties of the enamel composition are deteriorated. When less than 5 wt % of one or more of the MnO₂, MoO₃, Bi₂O₃, or NiO is included, a catalytic reaction on the surface of the enamel coating layer occurs less frequently. Accordingly, a cleaning performance of enamel may be deteriorated.

Additionally, the enamel composition may further include SiO₂ and/or B₂O₃. Silicon dioxide (SiO₂) is a ingredient that forms a glass structure. SiO₂ reinforces a skeleton of the glass structure and enhances chemical resistance of the enamel composition. The enamel composition may further include 20 or less wt % of SiO₂. When more than 20 wt % of SiO₂ is included, the ingredient interferes with the addition of transition metal oxides, thereby deteriorating cleaning performance.

Boron Oxide (B₂O₃) serves as a glass former and helps each ingredient of the enamel composition to melt uniformly. B₂O₃ enhances a coating performance by adjusting a coefficient of thermal expansion and fusion flow of the enamel composition. Twenty or less wt % of B₂O₃ may be included. When more than 20 wt % of B₂O₃ is included, the ingredient may interfere with the addition of transition metal oxides, thereby deteriorating a cleaning performance.

Next, the enamel composition may further include 1 to 20 wt % of aluminum oxide (Al₂O₃); 1 to 5 wt % of zirconium dioxide (ZrO₂); and 1 to 20 wt % of one or more of stannous oxide (SnO) or zinc oxide (ZnO). The above-described ingredients of Al₂O₃, ZrO₂, SnO, and ZnO may enhance durability of a weak alkali phosphate glass structure and may improve a hardness of the enamel surface. When more than 20 wt % of Al₂O₃ is included, melting temperatures go up and fusion flow increase, thereby reducing adhesion of the enamel coating layer. When more than 5 wt % of ZrO₂ is included, or when more than 20 wt % of SnO and/or ZnO are included, a glass structure may not be formed. Additionally, when a content of each ingredient is less than a minimum content thereof, durability of the enamel coating layer may be reduced.

The enamel composition may further include 2 or less wt % of titanium dioxide (TiO₂) to give a concealment force to a coating layer. When more than 2 wt % of TiO₂ is included, the ingredient may interfere with the addition of other ingredients, thereby deteriorating a cleaning performance, for example.

When both molybdenum trioxide (MoO₃) and bismuth trioxide (Bi₂O₃) are included, the enamel composition may include 2 or less wt % of any one of MoO₃ or Bi₂O₃. The ingredients of Mo and Bi may collide with each other in the phosphate-based enamel composition. Accordingly, a metallic crystal may be disposed in the enamel coating layer. Thus, when the ingredients of Mo and Bi are all included in the enamel composition, 2 or less wt % of any one of the ingredients may be included.

The enamel composition may have the above-described composition ratio, thereby making it possible to clean contaminants with no need to soak the contaminants in water.

Method of Preparing Enamel Composition

The method 100 for preparing an enamel composition according to embodiments may include providing the above-described materials for the enamel composition (110); melting the materials (120); and quenching the melted materials (130) to form the enamel composition. The materials may be sufficiently mixed and then melted. The materials may be melted in a range of temperatures of 1200 to 1400° C. Additionally, the materials may be melted for one to two hours. Then, the melted materials may be rapidly cooled by a chiller, for example, such as a quenching roller.

Cooking Appliance

An enamel composition according to embodiments may be coated on a surface of a target object. The target object may be all or a portion of a metallic plate, a glass plate, or a cooking appliance, for example. The enamel composition may be coated on an inner surface of a cavity of a cooking appliance, or on an inner surface of a door of a cooking appliance, for example.

Referring to FIG. 1 , a cooking appliance 1 according to an embodiment may include a cavity 11 in which a cooking chamber is formed, a door 14 that opens and closes the cooking chamber, at least one of heat sources 13, 15, 16 that supplies heat to the cooking chamber, a buffer layer 19, 20 coated on an inner surface of the cavity 11 or an inner surface of the door 14, and a coating layer 17, 18 formed by the enamel composition according to embodiments.

The cavity 11 may have a cuboid shape, a front surface of which is open. The heat sources 13, 15, 16 may include a convection assembly 13 that discharges heated air into the cavity 11, an upper heater 15 disposed at an upper portion of the cavity 11, and a lower heater 15 disposed at a lower portion of the cavity 11. The upper heater 15 and the lower heater 16 may be provided inside or outside of the cavity 11. The heat sources 13, 15, 16 may not include all of the convection assembly 13, the upper heater 15, and the lower heater 16. That is, the heat sources 13, 15, 16 may include any one or more of the convection assembly 13, the upper heater 15, and the lower heater 16.

Referring to FIGS. 2 and 3 , the enamel composition may be coated on an inner surface of the cavity 11 of the cooking appliance 1 or on an inner surface of the door 14 of the cooking appliance 1 through a dry process or a wet process, for example. The cavity 11 and the door 14 may include a metallic plate. The buffer layer 19, 20 may be formed on surfaces of the cavity 11 and the door 14, and the coating layer 17, 18 formed using the enamel composition according to embodiments may be coated on the buffer layer 19, 20.

The buffer layer 19, 20 may be formed into a coating layer having ingredients similar to those of the enamel composition according to embodiments. The buffer layer 19, 20 may include a ingredient having a coefficient of thermal expansion matching a coefficient of thermal expansion of a base metal plate and a ingredient having excellent adhesion to the base metal plate.

During the dry process, materials for the enamel composition may be dispersed in an organic binder, the mixed materials and organic binder may be milled in a ball mill, and a glass frit may be manufactured. During the wet process, materials for the enamel composition may be dispersed in water (H₂O) and pigment, the mixed materials, water (H₂O), and pigment may be milled in a ball mill, and a glass frit may be manufactured.

Then, the glass frit prepared according to the dry process or the wet process may be applied onto the buffer layer through a spray process, for example. The applied glass frit may be calcinated for 100 to 450 seconds in a range of temperatures of 600 to 900° C., and may be coated on the inner surface of the cavity 11 or the inner surface of the door 14 of the cooking appliance 1.

Hereinafter, embodiments will be described with reference to examples.

EXAMPLES

Method for Preparation of Enamel Composition

An enamel composition having a composition ratio described in the following Table 1 was prepared. Raw materials of each ingredient were sufficiently mixed for three hours in a V-mixer. Ammonium dihydrogen phosphate (NH₄H₂PO₄) was used as a raw material of P₂O₅. Sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃), and lithium carbonate (Li₂CO₃) were respectively used as raw materials for Na₂O, K₂O, and Li₂O. The mixed materials were sufficiently melted for one and a half hours at 1300° C. and were rapidly cooled in a quenching roller. Then a glass cullet was obtained.

For producing frits (powder), initial granularity of the glass cullet obtained through the above-described processes was controlled with the ball mill, was ground for about five hours using a jet mill, and then passed through a 325 mesh sieve (ASTM C285-88) such that a particle diameter of the glass cullet was limited to 45 μm or less.

TABLE 1 Comparative Component Embodiment example (wt %) 1 2 3 4 1 2 SiO₂ 13.36 13.03 14.28 14.8 0 1.26 P₂O₅ 22.96 19.98 21.9 24.06 52.59 51.26 B₂O₃ 8.4 6.61 12.04 9.3 0 0 Na₂O 0 0 4.13 4.73 1.86 1.86 K₂O 13.85 11.56 10.27 10.64 4.56 4.56 Li₂O 0.81 0.79 0.87 0.9 0 0 NaF 0 1.57 1.72 1.79 0 0 CaF₂ 1.61 0 0 0 0 0 MgO 0 0 0 0 7.24 7.24 Al₂O₃ 15.37 14.99 16.42 17.02 0 0 CaO 0 0 0 0 2.76 2.21 TiO₂ 0 0.8 0.88 0.91 1.86 2.48 MnO₂ 0 0 0 0 7.42 7.42 NiO 0.82 0 0 0 2 2 CuO 0 0 1.96 0 0 0 ZnO 9.91 16.51 0 0 0.93 0.93 ZrO₂ 3.88 2.18 2.39 4.29 0 0 MoO₃ 8.2 11.98 13.14 1.57 0 0 SnO 0.83 0 0 0.91 0 0 BaO 0 0 0 0 18.78 18.78 Bi₂O₃ 0 0 0 9.08 0 0

Preparation for Sample of Enamel Composition

A low carbon steel sheet having 200×200 mm and a thickness of 1 mm or less to be used for a sample was prepared. A buffer layer including ingredients in the following Table 2 was formed on the sheet. The buffer layer was manufactured in the same way that the above-described enamel composition was manufactured. A method of forming the buffer layer on the sheet is the same as a below-described method of forming an enamel coating layer.

Next, the frits, which were manufactured using the enamel composition according to Embodiments 1 to 4, and Comparative Examples 1 to 2, were sprayed onto the buffer layer with a corona discharge gun. A voltage of the corona discharge gun was controlled under the conditions of 40 kV to 100 kV, and an amount of the frits sprayed on the low carbon steel sheet was 300 g/m². The low carbon steel sheet onto which the frits were sprayed was calcinated at temperatures of 830° C. to 870° C. for 300 to 450 seconds to form a coating layer on one surface of the low carbon steel sheet. In this case, the coating layer was formed to have thicknesses of about 80 μm to 250 μm. By doing so, the sample was prepared according to Embodiments 1 to 4, and Comparative Examples 1 to 2.

TABLE 2 Component (wt %) Buffer layer SiO₂ 48.8 B₂O₃ 10.1 Na₂O 15 K₂O 10.7 Li₂O 4.2 NaF 6 TiO₂ 2.4 Co₃O₄ 1 NiO 0.5 Fe₂O₃ 0.8 MnO₂ 0.5

Experimental Example

Performance of the sample according to the above-described embodiments and comparative examples was evaluated as follows. Table 3 shows the results.

1. Cleaning Performance of Chicken Fat as Contaminant

One gram of chicken fat was thinly applied as a contaminant onto the surface of the sample, where a metallic substrate (100×100 (mm)) was coated with the enamel composition, with a brush evenly. Then, the sample to which the contaminant was applied was put into a thermostat and the contaminant was fixed for an hour in a range of temperatures of 250 to 290° C. After the contaminant was fixed, the sample was cooled naturally and was burned for an hour at a temperature of 350° C. Then, the hardened contaminant was cleaned with a kitchen scrubber for a frying pan, which was wet with room-temperature water, using a force of 3 kgf or less. Cleaned portions of the contaminated surface of the sample were uniformalized using a rod having a flat bottom and a diameter of 5 cm.

2. Cleaning Performance of Monster Mash

Cleaning performance of monster mash was evaluated using the same method as the above-described method. Frequency of back and forth cleaning motions made to the samples was measured and the frequency was defined as a frequency of back and forth cleaning motions. Table 3 shows indices of evaluation of cleaning performance.

TABLE 3 Frequency of back and forth cleaning motions Level 1~5 LV.5  6~15 LV.4 16~25 LV.3 26~50 LV.2 51~ LV.1

TABLE 4 Comparative Embodiment example 1 2 3 4 1 2 Cleaning LV.5 LV.5 LV.4 LV.5 LV.2 LV.1 performance of chicken fat Cleaning LV.5 LV.5 LV.5 LV.4 LV.1 LV.1 performance of monster mash

As shown in FIG. 4 , the embodiments disclosed herein have excellent cleaning performance. The comparative examples were less excellent in cleaning performance than the embodiments as the comparative examples had a composition less optimal than the composition of the embodiments.

Embodiments disclosed herein provide an enamel composition where contaminants, such as poultry fat, may be easily cleaned without being soaked in water. Further, embodiments disclosed herein provide a new enamel composition having a composition ratio that may cause no decrease in durability even though the enamel composition includes P₂O₅, and group I-based oxides.

To provide an enamel composition where contaminants, such as poultry fat or monster mash, may be cleaned without being put into water, an enamel composition according to embodiments may include 15 to 50 wt % of P₂O₅; 5 to 20 wt % of one or more of Li₂O, Na₂O, or K₂O; 1 to 5 wt % of one or more of NaF, CaF₂, or AlF₃; 1 to 35 wt % of one or more of MgO, BaO, or CaO; and 5 to 20 wt % of one or more of MnO₂, MoO₃, Bi₂O₃, or NiO.

To provide a new enamel composition having a composition ratio that causes no decrease in durability although the enamel composition includes P₂O₅, and group I-based oxides, an enamel composition according to embodiments may include 20 or less wt % of SiO₂, and 15 or less wt % of B₂O₃, and may further include 1 to 20 wt % of Al₂O₃; 1 to 5 wt % of ZrO₂; and 1 to 20 wt % of one or more of SnO or ZnO. To maximize cleaning performance, an enamel composition according to embodiments may include 2 or less wt % of any one of MoO₃ or Bi₂O₃ when the enamel composition includes both MoO₃ and Bi₂O₃.

The enamel composition according to embodiments may include a new phosphate-based glass composition and may be easily cleaned without being soaked in water. The enamel composition according to embodiments may further include additional ingredients, thereby causing no decrease in durability, and a content of some ingredients may be controlled, thereby maximizing cleaning performance. The enamel composition may also be coated onto a buffer layer formed on a base metal plate with no need to consider adhesion to the base metal plate.

Embodiments are described with reference to embodiments illustrated in the drawings. However, the embodiments are not limited to the embodiments and the drawings set forth herein. Further, various modifications may be made by one having ordinary skill in the art within the scope of the technical spirit. Furthermore, though not explicitly described during description of the embodiments, effects and predictable effects according to the configuration should be included in the scope.

It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

Spatially relative terms, such as “lower”, “upper” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “lower” relative to other elements or features would then be oriented “upper” relative to the other elements or features. Thus, the exemplary term “lower” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Embodiments of the disclosure are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the disclosure. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments.

Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art. 

What is claimed is:
 1. A cooking appliance, comprising: a cavity in which a cooking chamber is formed; a door that opens and closes the cooking chamber; at least one of heat sources that supply heat for heating an object to be cooked in the cooking chamber; a buffer layer coated on an inner surface of the cavity or an inner surface of the door; and a coating layer coated on the buffer layer and formed using an enamel composition, wherein the enamel composition comprises: 15 to 50 wt % of phosphorus pentoxide (P₂O₅); 5 to 20 wt % of one or more of lithium oxide (Li₂O), sodium oxide (Na₂O), or potassium oxide (K₂O); 1 to 5 wt % of one or more of sodium fluoride (NaF), calcium fluoride (CaF₂), or aluminum fluoride (AlF₃); 1 to 35 wt % of one or more of magnesium oxide (MgO), barium oxide (BaO), or calcium oxide (CaO); 5 to 30 wt % of one or more of manganese dioxide (MnO₂), molybdenum trioxide (MoO₃), bismuth oxide (Bi₂O₃), or nickel oxide (NiO); and 2 or less wt % of titanium dioxide (TiO₂), wherein when the enamel composition includes both MoO₃ and Bi₂O₃, 2 or less wt % of any one of MoO₃ or Bi₂O₃ is included.
 2. The cooking appliance of claim 1, wherein the enamel composition further comprises: 20 or less wt % of silicon dioxide (SiO₂).
 3. The cooking appliance of claim 1, wherein the enamel composition further comprises: 15 or less wt % of Bi₂O₃.
 4. The cooking appliance of claim 1, wherein the enamel composition further comprises: 1 to 20 wt % of aluminum oxide (Al₂O₃); 1 to 5 wt % of zirconium dioxide (ZrO₂); and 1 to 20 wt % of one or more of tin oxide (SnO) or zinc oxide (ZnO). 